There is a proposal for an electrophotographic image forming apparatus capable of forming fixed toner images having different glossiness in a recording sheet. The image forming apparatus includes an image forming device to form an image including an elastic transparent toner image and a viscously flowable transparent toner image (i.e., a transparent toner image viscously flowable when heated) adjacent to the elastic transparent toner image; and a fixing device to fix the two kinds of transparent toner images on a recording sheet upon application of heat and pressure thereto. The fixing device includes a high temperature fixing device and a heat/cool fixing device. The heat/cool fixing device applies heat and pressure to the image so that the elastic transparent toner image and the viscously flowable transparent toner image have the same strain, resulting in formation of transparent images having the same glossiness after the images are fixed. In contrast, the high temperature fixing device applies heat and pressure to the image so that the elastic transparent toner image and the viscously flowable transparent toner image have different strain, resulting in formation of transparent images having different glossiness after the images are fixed. It is described in the proposal that by using this image forming apparatus, invisible images (images having low visibility) consisting of transparent toner images having the same glossiness or visible images (images having high visibility) consisting of transparent toner images having different glossiness can be formed on demand. In addition, it is described in the proposal that by treating an invisible image, which consists of transparent toner images having the same glossiness and which is obtained by the heat/cool fixing device, using the high temperature fixing device, the invisible image can be changed to a visible image.
Recently, there is a need for a technique of producing a high value added color image, which includes an image portion having a relatively low glossiness and another image portion having a relatively high glossiness, using color toners and a transparent toner or a metallic toner. When the above-mentioned image forming apparatus is used for producing such a high value added color image, the following problems are caused.
Specifically, when unfixed toner images including an elastic transparent toner image and a viscously flowable transparent toner image are fixed by the high temperature fixing device of the above-mentioned image forming apparatus, the strains stored in the elastic transparent toner image and the viscously flowable transparent toner image are different from each other. In this regard, an elastic transparent toner image surrounded by a viscously flowable transparent toner image is prevented from moving in the horizontal direction, and the surface thereof is raised to release the strain stored therein, resulting in decrease of the glossiness. Since the elastic transparent toner image has a glossiness different from that of the viscously flowable transparent toner image, a visible transparent image can be formed.
Thus, it is necessary for the image forming apparatus to use two kinds of transparent toners and to form a first transparent toner image portion formed by an elastic transparent toner and a second transparent toner image portion, which is adjacent to the first transparent toner image portion and which is formed by a viscously flowable transparent toner, so that the resultant fixed first and second toner image portions have different glossiness, thereby forming a visible transparent image. Therefore, it is not possible for the image forming apparatus to form an image, in which a transparent toner image is formed on a desired image portion of an image to differentiate the glossiness thereof from those of other image portions of the image, in a recording sheet by using only one kind of transparent toner.
It is considered that the viscously flowable transparent toner image, which prevents the elastic transparent toner image from moving in the horizontal direction, is replaced with a viscously flowable color toner to produce such a high value added image as mentioned above. However, it is necessary to surround the image area formed of the elastic transparent toner with the color toner image. Since this is a positional restriction, it is not necessarily possible to form a desired high value added image.
The purpose of the image forming apparatus mentioned above is to incorporate a transparent image having a low visibility in a print image, wherein the transparent image includes information on the print image and has small glossiness difference such that a person can only just observe the transparent image. Namely, the difference in glossiness between the transparent toner images, which is caused by the roughened surface of one of the transparent toner images, is much lower than the desired glossiness difference in a high value added image.
For these reasons, the inventors recognized that there is a need for an image forming system capable of producing a high value added color image including an image portion having a relatively high glossiness compared to other image portions.